The art of xerography, as disclosed in U.S. Pat. No. 2,297,691 to C. F. Carlson, involves the formation of a latent electrostatic image on a photoconductive substrate with subsequent development by use of a particulate electroscopic marking material known as toner. The toner is normally comprised of a thermoplastic resin which can be thermally fused so as to provide a permanent image. The toner can be fused directly to the photoconductive substrate as in the case of coated paper xerography or transferred to and fused to paper as in the case of plain paper xerography. The relative ease with which such an electrostatographic copy can be made has led researchers to look for methods whereby the copy can be used as a master for duplicating more of its kind.
The use of a xerographic plate as a lithographic master has been disclosed in U.S. Pat. No. 3,460,476. In this process, an electrostatic latent image is formed on the surface of a hydrophilic glass photoconductive plate and developed with a hydrophobic toner. Lithographic ink and a fountain solution are then applied to the plate whereby the ink will conform to the hydrophobic image in an imagewise configuration. Contacting the inked surface with a transfer sheet will then affect the transfer of the image on the master to the transfer sheet.
The above-described system is effective for making a large number of copies from an easily prepared lithographic master. However, the process is less desirable when only a few copies are needed due to the cost involved in preparing the master. In addition, this method requires the use of a fountain solution in conjunction with the lithographic ink. The use of a separate lithographic ink and fountain solution has proven problematical since such a process requires the sequential application of water and ink each time a print is made, and this repeated operation decreases the useful life of the plate. Trouble is also encountered through partial comingling or emulsification of the oil and water on the plate.
One of the proposed methods for overcoming some of the defects of conventional lithographic processes has been the use of water-in-oil emulsion inks. However, emulsion inks suggested for use in lithography generally suffer from certain drawbacks which have not made their use particularly feasible. Some such inks do not have the capacity to remain in an emulsive state, that is, they have a tendency to demulsify or break in the ink fountain or during storage. Others do not provide satisfactory prints, particularly where a large number of copies are to be produced.
It would be desirable and it is an object of the present invention to provide a method and composition for the duplication of original documents comprised of marking material dispersed in an imagewise manner upon a substrate.
An additional object is to provide such a composition and method which can be used to duplicate an image comprised of an organic polymeric marking material on a substrate.
A further object is to provide such a method whereby the master is prepared by electrostatographic imaging and the marking material is comprised of a thermoplastic resin and a pigment.
An additional object is to provide such a method in which a plain or coated paper xerographic copy can be used as a lithographic master.
A further object is to provide such a process in which no fountain solution is required.